The Da Vinci Code is about Jesus. The Historian is about vampires.
The Da Vinci Code is for people who have never once in their lives questioned Christ's divinity. The Historian is about vampires.
The protagonists of The Historian are thoughtful university types who enjoy good food and Europe. The Da Vinci Code's protagonists take the better part of an hour to figure out a sixth grade-level anagram.
The Historian's two central characters are realistic, flawed, interesting human beings. The Da Vinci Code's two lead characters are described as Harrison Ford-like and a knock-out, leggy French chick ... who, we are asked to accept, has a brain (see last point.)
The Historian: a horrifying Wallachian Count. The Da Vinci Code: an effeminate English Lord.
The Historian is creepy, and at times, very funny. The Da Vinci Code has a lot of exciting driving scenes.
(I read all of The Da Vinci Code this past summer. I am only on page 60 of The Historian.)